This invention relates to troweling apparatus for applying a protective coating of cement mortar or similar material to the inside surface of pipe.
The application of mortar or similar plastic material to the interior surfaces of pipelines already in place has been used for a number of years to recondition and protect the interior surface of pipes.
A successful type of coating applicator and troweling apparatus is disclosed in U.S. Pat. No. 3,333,311. That patent discloses a mortar-dispensing machine which precedes a frusto-conical trowel towed by the dispenser. The trowel in that patent is made up of a plurality of arcuate leaves which overlap circumferentially to form an expandable troweling edge at the larger and trailing end of the trowel. The leaves are urged outwardly by an annular coil spring inside the trowel. The patented trowel does a good job in pipelines of relatively small diameter, but difficulties are encountered in trying to use the patented pipe trowel to line pipes having an interior diameter of more than about 24 inches. The annular compression spring is so large and heavy that it increases the weight of the trowel and causes excessive pressure on the lining material at the bottom of the pipe, thereby resulting in uneven lining thickness. Another disadvantage is that the weight of the spring causes it to sag away from the trowel leaves across the upper portion of the pipe. Thus, the spring does not exert an equal pressure on the upper leaves in the trowel, further aggravating the uneveness of the lining thickness. Attempts to avoid the problems caused by the large annular spring have included the use of individual short springs at each joint where the leaves overlap. This was unsuccessful in large diameter trowels, because the relatively large span in the arc of the individual leaves causes buckling of the leaves so that they do not exert a uniform pressure around the entire circumference of the troweling edge. To alleviate that difficulty, the number of leaves on the troweling device were increased to shorten the arcuate span of each individual leaf to avoid buckling. The increased number of leaves is undesirable because it requires more parts, and each overlap between adjacent leaves tends to leave a ridge in the coating material after the trowel passes. This is undesirable because the purpose of the lining operation is to provide the smoothest possible surface for minimum friction losses when the lined pipe is placed in service.
The present invention eliminates the weight and sag problem of the large annular spring, and the tendency for the trowel leaves to buckle without increasing the number of leaves.